This is a really simple formula and its meaning for me is that a little knowledge (little k) is a Dangerous Thing (big DT).
If you don’t believe this formula, ask yourself this question: Would you be happy to be diagnosed by a first year medical student if they didn’t have a consultant standing beside them during your examination? To put it a slightly different way, have you ever read through a family medical encyclopaedia? What did you experience as you read through all of the different illnesses and their symptoms? I’d be willing to bet that having read only a few pages, you started to notice that you had some of the symptoms and that you started to wonder how it was that you hadn’t spotted them before.